Various communication systems provide a communication path between terrestrial data processing systems through a satellite system. The earliest satellite systems, known as "bent pipe" satellites, echoed signals between terrestrial transmitters and receivers. These systems required a significant number of satellites and terrestrial transmitters and receivers.
More recently, satellite systems route communication between satellites, thus requiring fewer terrestrial stations. Inter-satellite routing methods include circuit switching and packet switching. Circuit switch routing defines a communication path from source to destination and maintains that channel throughout the communication. Circuit switching is inherently inefficient since the channel is dedicated to the communication regardless of the amount of data being transmitted.
Packet switch routing, also known as virtual circuit switching, allocates the communication paths for each segment("packet") of the communication rather than the entire communication. Thus, a path can be used by multiple terminals, since each only uses that path during data transmission. U.S. Pat. No. 5,600,629 discloses an example packet switch routing system.
Communication paths can be either static or dynamic. Static communication paths are determine at the beginning of a communication session and used throughout that session. Since the routing is static, all packets associated with the communication follow the same communication path. Static routing provides a simple mechanism to route messages. However, it fails to take to advantage of changing loads and resource availability.
Dynamic routing transfers data packets based on system resource availability. Thus, each data packet may traverse the network through a different route. However, dynamic routing that bases route determination on system resource that are available at the beginning of a transmission fail to take advantage of resource changes that occur during the transmission of the data packets.
Satellite communication protocols often vary substantially from their terrestrial counterparts. Thus, significant time and computations are required to convert terrestrial communication protocols to satellite communication protocols. A need exists for a satellite system that uses a protocol substantially similar to terrestrial protocols.
Addressing in satellite systems is either based on satellite locations or fixed geographic locations.
An Earth fixed cell addressing methods is disclosed in U.S. Pat. No. 5,408,237, Patterson, et al. Such addressing reduces the computations during the transfer or "hand-off" of terrestrial links.
Non-geostationary satellites must hand-off terrestrial communication links before the satellite's coverage area moves beyond the terrestrial gateway. U.S. Pat. No. 5,625,867 discloses a hand-off method based in part on the quality of service. Efficient utilization of over-lapping can be maximized by determining hand-off based not only on quality of service but also on a variety of system parameters. Thus, a need exists for a satellite system to base the hand-off decision on a variety of system parameters.
Existing satellite systems coordinate between two overlapping satellites. The satellite currently linked to the terrestrial station acts as the master and the other over-lapping satellite acts as a slave. As the master, a satellite determines when to hand-off a communication link. Over-lapping satellites require more coordination when the complexity of determining efficient hand-offs increases, and when the number of overlapping satellites increase, and when the number of hand-offs increases. Thus, a need exist for a satellite system with a coordinating satellite that determines when hand-off should occur and between which satellites the hand-off should occur. Such a control satellite minimizes the computations required to determine the optimal load balancing between over-lapping satellites within the static cell.